Field of the Invention
Embodiments of the invention generally relate to epitaxial lift off (ELO) processes.
Description of the Related Art
One phase in device fabrication involves handling and packaging of thin films used in photovoltaic, semiconductor, or other devices. Such thin film devices may be manufactured by utilizing a variety of processes for depositing and removing materials onto a wafer or other substrate. FIG. 1 shows a typical wafer 101 that is used as a growth substrate. Wafer 101 has a flat cut 103 which indicates its crystallographic orientation. This wafer is typically diced into a rectangular or square configuration 105. An edge of the rectangle or square 105 aligns with that of a flat cut 103. The edges of the rectangle or square 105 are either parallel or perpendicular to flat cut 103. This is referred to as an “on-axis” orientation.
One common technique for manufacturing thin film devices is known as the epitaxial lift off (ELO) process. In an ELO process, an epitaxial material or film, often containing multiple layers, is formed on a sacrificial layer which is deposited on the growth wafer. The sacrificial layer is etched away and the epitaxial film is separated from the growth wafer. The isolated thin epitaxial film may then be further processed and incorporated into a final product, such as in photovoltaic, semiconductor, or other devices.
The sacrificial layer is typically very thin and is usually etched away via a wet chemical process. The speed of the overall process may be limited by the lack of delivery or exposure of reactant to the etch front, which leads to less removal of by products from the etch front. The etching process is partially a diffusion limited process, and if the films were maintained in their deposited geometries, a very narrow and long opening would form to severely limit the overall speed of the process. To lessen the transport constraint of the diffusion processes, it may be beneficial to open up the resulting gap created by the etched or removed sacrificial layer and bending the epitaxial film away from the growth wafer. A crevice is formed between the epitaxial film and growth wafer, providing greater transport of species both towards and away the etch front. Reactants move towards the etch front while by-products generally move away from the etch front.
Furthermore, the epitaxial films grown on nearly rectangular or square growth wafers, or other wafers having sharp angles, are especially susceptible to developing corner cracks during the ELO processes. Basically, the diagonal corner regions experience similar or a slightly slower etch rate compared to the straight edges. This results in the corners of the sacrificial layer 201 becoming narrower, more pointed and pronounced, as depicted in FIG. 2. These narrower corners holding down the epitaxial film endure correspondingly increased stresses when the epitaxial film is being separated.
Therefore, there is a need for a method to remove epitaxial films from the growth wafers without tearing the films during an ELO process, as well as to maintain or increase the throughput of the process.